The present invention relates to linear rectifiers in general, and specifically, to improved dynamic range through the rectification of very small oscillatory input signals.
The prior art teaches the coupling of an oscillatory signal to a solid state diode half-wave rectification, or to a diode bridge or transformer/diode combination for a full-wave rectification. In diodes which are commonly utilized, there is a temperature-dependent voltage which is necessary as a forward bias before the diode will begin conduction. As can be seen by reference to FIG. 1, this small voltage, called the Cutin voltage, is on the order of approximately 0.6 volts, although it can, and does, change with the temperature of the diode. As can be seen with a bias of between 0.4 and 0.6 volts, the diode begins conduction in a non-linear manner.
It is known that a sharp knee can be provided by employing operational amplifier and diode combinations, such as described in Handbook of Operational Amplifier Applications by Burr-Brown Research Corporation. Unfortunately, these configurations are not acceptable, in that they do not have a suitable bandwidth for signals in excess of 5 MHz, for example, as required in ultrasonic testing.
Additionally, in the ultrasonic testing field, it is desirable to track the envelope amplitude of the ultrasonic signal on virtually a cycle-by-cycle basis for both large and small signals. Because of the non-linear conduction threshold of the prior art devices, this has heretofore been very inaccurate, and many times impossible.